How to optimize old meshes?
You can use the Decimate modifier to optimize the mesh and it should preserve UVs.

Select your object in Object mode

Add Modifier Decimate

In the modifier settings, select Planar and set a low angle limit (1 degree is enough if your mesh is truly flat).


Apply the modifier when you're done. Check the face count to see how many polygons you have now.


Note: The Decimate modifier affects the whole object